U.S. Pat. No. 5,399,037 ("'037 application") which is incorporated herein by reference, discloses a logging-while-drilling apparatus as shown in FIG. 1. This apparatus comprises a subsystem 200 which forms part of a bottom hole assembly (BHA) of a drill string in a borehole (not shown).
The subsystem 200 includes a section of tubular drill collar 202 having mounted thereon a transmitting antenna 205, a receiving antenna 207, and receiving electrodes 226, 227,228 and 235. In the illustrated subsystem the transmitting antenna 205 comprises a toroidal antenna having coil turns wound on a ferromagnetic toroidal core that is axially coincident with the axis of the drill collar 202. The core may have an elliptical or rectangular cross-section, although other shapes can be used. The purpose of this toroidal transmitter is to act as a primary of a transformer and to induce a voltage along the drill collar. The drill collar and the formations represent a one-turn secondary winding. If the transmitter is excited with a drive voltage V.sub.T and the transmitter toroid has NT turns, then the voltage induced along the drill collar will be V.sub.T /N.sub.T. That is, the voltage difference between the drill collar above the transmitter and the drill collar below the transmitter will be V.sub.T /N.sub.T. The resultant current travels in a path that includes the drill string and the formations (as well as the borehole fluid which is assumed to have substantial conductivity). The receiving electrodes 226, 227 and 228 are button electrodes mounted in a stabilizer 220, and electrode 235 is a ring electrode. The receiving antenna 207 is another toroidal coil antenna. The toroidal receiver measures the axial current flowing through the drill collar. If the receiver toroid contains NR turns and the current in the drill collar is I, the current flowing through the receiver winding into short circuit will be I/N.sub.R.
An annular chassis 290, which contains most of the electronics, fits within the drill collar 202. In this configuration, the drilling mud path is through the center of the chassis, as illustrated by arrows 299. The chassis 290 has a number of slots, such as for containment of batteries (at position 291) and circuit boards (not shown). In this configuration the circuit boards are in the form of elongated thin strips, and can accordingly be planar. Other circuit board configurations or circuit packaging can be utilized. The transmitting toroidal antenna 205 [which can also be utilized in a communications mode as a receiver] is supported in a suitable insulating medium, such as "VITON" rubber 206. The assembled coil, in the insulating medium, is mounted on the collar 202 in a subassembly which includes a protective tapered metal ring 209, that is secured to the collar surface by bolts (not shown). The antenna wiring, and other wiring, is coupled to the annular circuit assembly via bulkhead feed-throughs (not shown). The receiving toroidal coil antenna 207 is constructed in generally the same way, although with more coil turns in the described configuration, in insulating medium 211, and with protective ring 213. The receiving ring electrode 235 is also mounted in an insulating medium such as a fiberglass-epoxy composite 236, and is held in a subassembly that includes tapered ring 237, which can be integrated with the protective ring for the receiving antenna 207.
The three button electrodes 226, 227 and 228 are provided in stabilizer blade 220 which may have, for example, a typical straight or curved configuration. The stabilizer blades are formed of steel, integral with a steel cylindrical sleeve that slides on the drill collar 202 and abuts a shoulder 203 foraged on the drill collar. The stabilizer is secured to collar 202 with lock nuts 221. The blades can be undersized to prevent wear of the electrodes. The button faces can have generally cylindrical curvatures to conform to the stabilizer surface or can have flat faces with surfaces that are slightly recessed from the stabilizer surface shape. These electrodes span only a small fraction of the total circumferential locus of the borehole and provide azimuthal resistivity measurements. Also, these electrodes have a vertical extent that is a small fraction of the vertical dimension of the stabilizer on which they are mounted, and provide relatively good spatial resolution resistivity measurements. In the described configuration, the top portion of each electrode is surrounded by an insulating medium, such as "VITON" rubber, which isolates the electrode surface from the surface of the stabilizer blade 220. A fiberglass epoxy composite can be used around the base of the electrode. The electrodes 226, 227 and 228 provide a return path from the formations to the collar 202 (of course, when the AC potential reverses the current path will also reverse), and the current is measured to determine lateral resistivity of the region of the formation generally opposing the electrode. The electrodes 227 and 228 are respectively further from the transmitter than the electrode 226, and will be expected to provide resistivity measurements that tend to be respectively deeper than the measurement obtained from electrode 226. The electrodes are mounted in apertures in the stabilizer 220 that align with apertures in the drill collar 202 to facilitate coupling of the electrodes to circuitry in the annular chassis 290.
The '037 Patent also discloses further apparatus as shown in FIG. 2, which has toroidal transmitter T1 and a ring electrode R on a conductive body 1202 which is like the drill collar 202 in a logging-while-drilling setup of the general type shown in FIG. 1. A further toroidal transmitter, T2, also called a lower transmitter, is located near the drill bit 15. For this example, the ring electrode is about equidistant from the transmitters. A receiver monitor toroid M.sub.0 is located near the ring electrode R to monitor the axial current flowing along the conductive body at the position of the ring electrode R. A lower receiving or monitoring toroid M.sub.2 is located adjacent the lower transmitter T.sub.2.
The axial current which is induced by T1 is linear with respect to the voltage induced on the drill collar and approximately inverse to the resistivity of the earth formation surrounding the tool. The axial current which is induced by T2 is linear with respect to the voltage induced on the drill collar by T2 and approximately inverse to the resistivity of the earth formation surrounding the tool. The net axial current which flows along the drill collar at any point is the linear superposition of the induced current from T1 and T2. For the voltage of T2 to be adjusted so that the net axial current flowing through the monitor toroid M0 is zero will require that the current induced by T2 be opposite in phase to the current induced by T1, so that when the upper transmitter is driving current down the tool, the lower transmitter is driving the current up, and vice versa. All of the current leaving the tool between the lower transmitter and the monitor returns to the tool below the lower transmitter while all of the current leaving the tool between the upper transmitter and the monitor returns to the tool above the upper transmitter. This has the effect of isolating the region of the tool above the monitor from the region of the tool below the monitor since no current flows between them, either on the collar or through the formation. As a result, the resistivity determined from the ring current more accurately represents the resistivity of formations surrounding the ring R.
A similar result can be obtained by energizing the transmitters separately and computing a compensated ring current (the upper position is designated 1, the lower position is designated 2, and the center position is designated 0). The ring and toroid currents when the upper transmitter is operated at an arbitrary but fixed voltage are R.sub.1, M.sub.01, and M.sub.21 and the ring and toroid currents when the lower transmitter is operated at the same voltage are R.sub.2, M.sub.02, and M.sub.12. Consider a compensated current of the form: ##EQU1## In equation (1a), the ratio ##EQU2## is the adjustment factor for the lower transmitter to achieve the condition of zero axial current at MO. The expression ##EQU3## is the ring current for the condition of zero axial current. The prefactor, 1/M.sub.02 corresponds to a fixed voltage at T1 and a voltage at transmitter T2 of ##EQU4## The preferred factor is, 1/M.sub.21 where M.sub.21 is the current produced by the upper transmitter measured at the lower monitor toroid M2 (which is at substantially the same position as the lower toroidal transmitter T2). In this case, the compensated current is given by R.sub.c =1/M.sub.21 (M.sub.02 R.sub.1 +M.sub.01 R.sub.2).
The two terms in equation (1a) add. This is due to the fact that operating the two transmitters in opposition in order to achieve a zero axial current at the monitor toroid causes an increase in the ring current. That is, when the upper transmitter drives a current down the mandrel, current flows out of the ring. Similarly when the lower transmitter drives current up the mandrel, it also causes current to flow out of the ring. The implication of this processing on the noise is that, since the terms add, the noise in the output is not amplified as would be the case if one took a small difference between two large numbers.
The application of these teachings to a wireline logging tool is also disclosed in the '037 Patent.
U.S. Pat. No. 5,200,705 (incorporated herein by reference) also discloses a logging-while-drilling resistivity measurement using a multi electrode array to identify dipping beds in underground formations.
It is an object of the present invention to provide a method and apparatus in which focusing of signals can be achieved using software to reduce borehole effects and shoulder bed effects. It is also an object of the invention to provide good thin bed resolution.